fix: write duplication migration settings to identical_code and similar_code (#2721)

## Summary
- Config migrator was writing `enabled` and `threshold` to
`[smells.duplication]`, which only supports `nodes_threshold` and
`filter_patterns` — these settings were silently ignored
- Now writes `enabled` and `threshold` to `[smells.identical_code]` and
`[smells.similar_code]` separately
- Uses threshold of 15 (the qlty default already in effect) to avoid
changing behavior for existing customers

Closes #2717

## Test plan
- [x] Unit tests in `qlty-config` pass (`cargo test -p qlty-config
migration`)
- [x] Integration tests pass (`cargo test -p qlty config_migrate_tests`)
- [x] `cargo check` passes

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-authored-by: Claude Opus 4.6 <noreply@anthropic.com>

Author: davehenton

qlty-cli/tests/cmd/config/migrate/checks.stdout

@@ -40,7 +40,9 @@ threshold = 5
 [smells.function_complexity]
 threshold = 6
 
-[smells.duplication]
-enabled = true
-threshold = 30
+[smells.identical_code]
+threshold = 15
+
+[smells.similar_code]
+threshold = 15
 

qlty-cli/tests/cmd/config/migrate/exclude_patterns.stdout

@@ -19,6 +19,9 @@ threshold = 4
 [smells.function_complexity]
 threshold = 5
 
-[smells.duplication]
-threshold = 22
+[smells.identical_code]
+threshold = 15
+
+[smells.similar_code]
+threshold = 15
 

qlty-cli/tests/cmd/config/migrate/match_multiple.stdout

@@ -49,6 +49,9 @@ threshold = 4
 [smells.function_complexity]
 threshold = 5
 
-[smells.duplication]
-threshold = 22
+[smells.identical_code]
+threshold = 15
+
+[smells.similar_code]
+threshold = 15
 

qlty-cli/tests/cmd/config/migrate/match_path.stdout

@@ -45,6 +45,9 @@ threshold = 4
 [smells.function_complexity]
 threshold = 5
 
-[smells.duplication]
-threshold = 22
+[smells.identical_code]
+threshold = 15
+
+[smells.similar_code]
+threshold = 15
 

qlty-cli/tests/cmd/config/migrate/match_path_plugin_name.stdout

@@ -45,6 +45,9 @@ threshold = 4
 [smells.function_complexity]
 threshold = 5
 
-[smells.duplication]
-threshold = 22
+[smells.identical_code]
+threshold = 15
+
+[smells.similar_code]
+threshold = 15
 

qlty-cli/tests/cmd/config/migrate/match_url_filename.stdout

@@ -45,6 +45,9 @@ threshold = 4
 [smells.function_complexity]
 threshold = 5
 
-[smells.duplication]
-threshold = 22
+[smells.identical_code]
+threshold = 15
+
+[smells.similar_code]
+threshold = 15
 

qlty-cli/tests/cmd/config/migrate/match_url_plugin_name.stdout

@@ -45,6 +45,9 @@ threshold = 4
 [smells.function_complexity]
 threshold = 5
 
-[smells.duplication]
-threshold = 22
+[smells.identical_code]
+threshold = 15
+
+[smells.similar_code]
+threshold = 15
 

qlty-cli/tests/cmd/config/migrate/missing_codeclimate_yml_fetch.stdout

@@ -34,6 +34,9 @@ threshold = 4
 [smells.function_complexity]
 threshold = 5
 
-[smells.duplication]
-threshold = 22
+[smells.identical_code]
+threshold = 15
+
+[smells.similar_code]
+threshold = 15
 

qlty-cli/tests/cmd/config/migrate/no_match.stdout

@@ -41,6 +41,9 @@ threshold = 4
 [smells.function_complexity]
 threshold = 5
 
-[smells.duplication]
-threshold = 22
+[smells.identical_code]
+threshold = 15
+
+[smells.similar_code]
+threshold = 15